Electrostatic transducer



June 9, 1964 Filed Sept. 25, 1961 G. A. BRETTELL ELECTROSTATIC TRANSDUCER N T R m ea e e a e e e 6 a e e a a e a a a e a a 0 a 0 3 Sheets-Sheet 1 6502654. BET/TEL IN VEN TOR.

BY awa June 9, 1964 G. A. BRETTELL 3,136,857

ELECTROSTATIC TRANSDUCER Filed Sept. 25, 1961 3 Sheets-Sheet 2 xx HE @MKRQ] w U a H P Q 1 N VEN TOR.

ATTORNEY June 9, 1964 A. BRETTELL ELECTROSTATIC TRANSDUCER 3 Sheets-Sheet 3 Filed Sept. 25, 1961 m IW I-HH Q g Q 6E W & wNTU WWW m m w w 650/9654 Ea /m L IN VEN TOR.

ATTORMF) United States Patent C) l 3,136,867 ELECTROSTATIC TRANSDUCER George A. Brettell, Redwood City, Calif., assignor to Ampex Corporation, Redwood City, Calif., a corporation of California Filed Sept. 25, 1961, Ser. No. 140,551 3 Claims. (Cl. 179 111 The present invention relates to transducers and, more particularly, to an electrostatic transducer.

In general, transducers encompass devices for transforming one form of energy into a second form of energy. Within the scope of the present invention, the term transducer is used to define a means for changing electrical energy into energy taking the form of pressure variations in a fluid, or vice versa. In the past such devices have been generally electromagnetic in nature and have been the combination of a current carrying coil and a magnetic core structure with one being moved through its influence on the other to make the energy transformation. combination of coil and core are, for most applications, bulky and heavy where the output power requirements are large and a wide band of frequency response is de-- sired, in addition to having other limitations and disadvantages well-known in the art.

Another type of transducer has been developed to overcome some of the inherent disadvantages of the electromagnetic type, as previously discussed, and is based on electrostatic principles. Such electrostatic transducers, however, have been limited with respect to power output capability, particularly in the lower range of frequency response, because of the narrow spacing between the active elements thereof. In further explanation, consider the operation of an electrostatic transducer having a flexible diaphragm mounted between two rigid though acoustically transparent plates. With a given spacing, only a given voltage may be applied due to breakdown of the air dielectric, and thus a limit to the force which can be developed is established. Application of this class of transducer has therefore been limited to designs that can accept this limitation. In particular this limitation is more noticeable at low audio frequencies, since it is desirable to totally enclose the back radiation with a reason ably sized enclosure, and this in turn implies that high forces be developed to compress and rarefy the enclosed air.

The foregoing will be more apparent in considering a system, by way of example, for converting audio frequency electrical energy into acoustical energy. It is axiomatic in the art that for equal power output over the audio frequency range a much greater mass of air must be moved at the lower frequencies and this is the reason that low frequency loudspeakers are much larger and more heavily constructed than high frequency.loudspeakers. It is to be noted that the foregoing problems are more readily solved with low power speakers, such as earphones, where the power output at low frequencies is not too critical because of the closed system in close proximity to the ears of the hearer. Also, compensation may be made by suitable design of the frequency response characteristic of the audio amplifier that drives the transducer, or by the use of an acoustic absorbent material, or both. In either instance the low power output limitation is the result of the limited maximum excursions demanded of the flexible diaphragm.

It is therefore an object of the present invention to provide a new and improved transducer.

Another object of the invention is to provide a new and improved electrostatic transducer.

Still another object of the invention is to provide an electrostatic transducer having a greater power output The 3,136,867 Patented June 9, 1964 capability for a given frontal area than has heretofore been possible.

A further object of the invention is to provide a simple electrical-to-acoustical transducer wherein the required backing chamber volume is substantially reduced over that of the prior art for equal frequency response characteristics.

A still further object of the invention is to provide a simple, multi-purpose, and sensitive transducer.

According to the present invention a plurality of flexible diaphragms is mounted in a suitable system of stacked frames with rigid plates interleaved therebetween.

Both the diaphragms and plates are of conductive ma terials and plates are provided with a plurality of apertures. With such an arrangement, alternate ends of a center-tapped winding of a transformer are respectively connected to successive plates. to the diaphragms by connecting successive diaphragms to opposite terminals of a center-tapped source of unidirectional voltage with the center tap of the source con nected to the center tap of the Winding.

By applying an alternating input signal to the transformer, the diaphragms are each subjected to electrostatic forces in the same direction with the direction being reversed for all diaphragms for successive alterations of the signal. With the same type of element, such as the rigid apertured plates, at both ends of the laminar structure, the result is that the force contribution of each diaphragm is additive to that of the others. The increase in force is provided by the laminar type of construction without difference in displacement from diaphragm to diaphragm, except at very high frequencies, where provision must be made for wave propagation velocity for stacks having a large number of elements.

With minor variations in the mounting frames, and/or Other objects and advantages of the present invention will be readily apparent from the following description and claims, considered together with the accompanying drawing, in which:

FIGURE 1 is a schematic view of a simple electrostatic transducer of the prior art;

FIGURE 2 is a perspective view, partly broken away, of a transducer in accordance with the present invention;

FIGURE 3 is a partial cross section view of the invention of FIGURE 2 showing suitable electrical connections;

FIGURE 4 is a cross section of-the invention of FIG- URE 2 as mounted in a wall of a backing chamber;

FIGURE 5 is an exploded View of an embodiment of the present invention; 1

FIGURE 6 is a schematic view of the invention of FIGURES; and

FIGURE 7 is an illustration of another manner of applying variable voltages to rigid plates of FIGURE 3 for high frequency applications.

Referring now to the drawings in detail, FIGURE 1 in particular, there is illustrated a typical electrostatic transducer 11 of the prior art. Thus, a flexible diaphragm 12, such as Mylar with a conductive coating 13 of silver or the like, is provided as a principal element. Respectively mounted parallel to and at either side of the diaphragm 12 are rigid conductive plates 14 and 15, each having a plurality of apertures 17, 18, respectively, therein. By confining the periphery of the diaphragm 12 and plates 14 and 15 with suitable structure (not shown), any movement of the diaphragm results in a corresponding movement of fluid, such as air, present between diaphragms in directions transverse to the plates.

Suitable bias is supplied To provide a controlled movement of the referenced fluid, an electrical circuit 19 is provided to assert a pushpull component of electrostatic forces on the diaphragm 12. As illustrated, the circuit 19 comprises a transformer 21 having one winding 22 connected between two terminals 23 and 24 and a second winding 26 having a center tap 27. One lead 28 of the center-tapped winding 26 is connected to one plate 14 and another lead 29 is connected to the plate 15. To complete the electrical connections the combination of a bias supply 31, such as a series of batteries as shown, and a resistor 32 is connected between the center tap 27 and the diaphragm 12.

With an alternating signal impressed between the two terminals 23 and 24, the two rigid plates 17 and 18 are given opposite potentials with respect to the center tap during each alternation of the signal. Because of the bias potential of the diaphragm 12 and the connection of the diaphragm to the center tap 27, a net electrostatic force is developed on the diaphragm in a direction corresponding to the polarity of the input signal and the result is a movement of the diaphragm corresponding to the amplitude and polarity of the signal.

An analytical consideration of the above-described structure shows that the net force applied to the diaphragm 12 by the two plates 17 and 18 is inversely proportional to the square of the distance between the two plates. Thus, the force increases as the distance is decreased for constant values of signal and polarizing potential. Also, such considerations indicate that the force on the diaphragm 12 is directly proportional to the potential applied between the plates 17 and 18 times the polarizing voltage of the diaphragm. The foregoing factors are not compatible in practice when the lower frequencies of the audio range are to be converted in an enclosure. The foregoing is factual because considerable force must be exerted by the diaphragm and, if the potentials are increased with a given spacing, electrical breakdown occurs. If, with the increased potentials, the spacing is increased to avoid breakdown, the force developed is not increased.

Now, to illustrate a structure for overcoming the foregoing limitations with respect to electrostatic transducers, in accordance with the present invention, reference is made to FIGURE 2 of the drawings. In such figure three rigid conductive plates 41, 42, and 43 are respectively mounted in parallel and spaced-apart relation within suitable frames 46, 47, and 48. In the two spaces, thus provided, between the successive plates 41, 42, and 43 there are respectively disposed two thin flexible and slightly conductive diaphragms 51 and 52. The rigid plates 41, 42, and 43 are respectively provided with a plurality of apertures 53, 54, and 55, similar to the previously described plates 17 and 18. Also, the diaphragms 51 and 52 may be similar to the previously described diaphragm 12, but having less conductivity. The foregoing elements are clamped together in a suitable manner (not shown) to provide a unitary structure.

Electrical connections are then made to the active elements (plates and diaphragms) described above in the manner shown in FIGURE 3. Thus, alternate plates 41, 42, and 43 are directly connected to opposite terminals 57 and 58 of one winding 59 of a transformer 61; that is, the two outermost plates 41 and 43 are connected to one terminal 57 and the intermediate plate 42 is connected to the otherterminal 58. A second winding 62 of the transformer 61 has two terminals 63 and 64 to which the input signal is applied. Bias is applied, for example, to the two diaphragms 51 and 52 by two series-connected batteries 66 and 67 such that one polarity is impressed on one diaphragm 51 and the opposite polarity is impressed on the other diaphragm 52. To complete the electrical connections the mid-point 68 between the batteries 66 and 67 is connected to ground, as is a center tap 69 of the winding 59, or the mid-point may be directly connected to the center tap. For convenience of construction the plates 41, 42, and 43 extend into suitable slots (not num- 4- bered) of the frames 46, 47, and 48, respectively, and the leads between the winding 59 and the plates extend through communicating openings 71, 72, and 73, respectively.

By connecting a source (not shown) of audio signals to the terminals 63 and 64 of the transformer 61, the two diaphragms 51 and 52 are subjected to forces in the same direction. As an aid in understanding the foregoing, it is to be noted that the two diaphragms 51 and 52 are respectively connected to oppositely polarized terminals of the sources 66 and 67 of bias. In the absence of an audio signal at the fixed plates 41-43, there is no net force exerted on the diaphragms 51 and 52. Now by way of example, when an audio signal appears at the fixed plates 41-43 the balance of forces no longer exists because the signal is added to the bias at one plate 41, subtracted at the next adjacent plate 42, and added at the next adjacent plate 43. The net result is that the forces on the two diaphragms 51 and 52 are in the same direction and because of the stiffness of the trapped air in between, are additive. The amount of the force is then dependent upon value of the audio signal.

While the incompatibility between force and distance of the spacing between each diaphragm 51 and 52 and the associated pairs of adjacent plates 41, 42, and 42, 43, respectively, remains, the total force of the combination in the above-described laminar structure is increased by a factor of two. Thus, it is clearly apparent that the disadvantages of the three-element electrostatic transducer of the prior art have been overcome and it is now possible to use the advantages of the electrostatic transducer for applications that were previously impossible.

Because of the increased forces available with the laminar structure of the present invention, a reasonably dimensioned closed backing-chamber may be used for reproduction of sound over an extremely wide range of audio frequencies. With prior high power loudspeakers it is necessary to provide a large volume backing chamber because of the necessity of limiting the forces needed to compress the enclosed air. In order to overcome the foregoing, the prior art teaches the use of bafiles, or other substitutes for the closed backing chamber.

In addition to the foregoing, open backing chambers have been proposed, with or without the use of bafiles. One type requires that the loudspeaker be placed in the corner of a room with the backward dimensions from the moveable element of the transducer to the walls of the room having a specific relation with respect to the frequencies to be faithfully reproduced. Such arrangements are still compromises and fall far short of the ideal.

Since the present invention provides a greater force over the same area as prior art transducers, an audio loudspeaker of higher power output with a relatively small backing chamber is possible for reproduction of a wide band of audio frequencies. The reason for the foregoing is to be found in the fact that the force necessary to compress the air within the relatively small backing chamber is available with the transducer of the present invention, but not available with prior art transducers. Thus, with reference to FIGURE 4 a transducer 81, similar to that described with respect to FIGURES 2 and 3, is mounted in one wall of a closed rectangular box-like structure 82. Electrical connections to the various active elements of the transducer 81 are suitably made through the wall structure 82, though not shown for simplicity of illustration, in the manner of FIGURE 3. The relationship between the dimensions of the transducer 81 and those of the backing-structure 82 is dependent on the number of diaphragms of the transducer stack.

From the foregoing, then, it is readily apparent that the laminar structure of the invention permits a considerable reduction in the required dimensions of the backing chamber for faithful reproduction of the entire audio band of frequencies. To prevent resonances from interferring with the fidelity of the sound, the backing chamber 82 may be provided with acoustically absorbent material (not shown), either mounted on the Walls thereof, or inserted in the manner of a loose packing.

In addition to serving as an electrostatic-to-acoustic transducer, the structure shown in FIGURE 4 may also be used to provide controlled air impulses. According to such use the transducer 81 is confined within one end of an air column, such as that provided by a pipe or conduit (not shown), extended away from the backing-chamber structure 82. In such arrangement electrical connections are made in the manner of FIGURE 3 and a pulse of one polarity applied at the terminals 63 and 64 results in a flow of air in one direction with respect to the transducer 81 while a pulse of the opposite polarity causes an air flow in the opposite direction. It will be apparent that the foregoing structure and connections provide an electrical output at-the terminals 63 and 64 in response to diiferences in air pressure upon the transducer 81.

While the foregoing transducer 81 is useful as a pressure transducer, a minor variation in the frame structure holding the active elements and in the electrical connections thereto provides an ability to displace a large volume of air and thus increase the volume of delivery rather than pressure. Thus, referring to FIGURE 5, a transducer 91 is provided having three apertured rigid plates 92, 93, and 94 with two interleaved flexible diaphragms 96 and 97. Frames 101 and 102 of the two rigid plates 92 and 94 at the outermost portions of the transducer 91 are three sided with the open portion of each facing in the same direction while a similar frame 103 of the intermediate rigid plate 93 is disposed with the open portion facing in the opposite direction. Since the flexible diaphragms 96 and 97 require support on all sides, four sided frames 106 and 107 are respectively provided. To complete the transducer 91, top and bottom cover plates 108 and 109 are disposed adjacent to the respective end plates 92 and 94. The foregoing elements are then suitably clamped together, as by bolts (not shown) extended through the frame elements.

Electrical connections to the apertured plates 92, 93, and 94 and to the diaphgrams are then made as shown in FIGURE 6. A transformer 111 has a first winding 112 connected between two terminals 113 and 114 and a second, center-tapped, winding 116. One lead 117 of the center-tapped winding 116 is connected to the two outer rigid plates 92 and 94 of the stack and a second lead 118 is connected to the intermediate plate 93. Bias is applied to the diaphragms 96 and 97 by connecting one terminal of a source 121 of direct current, such as the battery shown, to both of the diaphragms and the other terminal of the source directly to the center tap of the winding 116, or, as shown, through ground connections.

In accordance with the foregoing structure, a signal coupled to the terminals 113 and 114 of the transformer 111 results in electrostatic forces being applied to the two diaphragms 96 and 97 in opposite directions. In such respect, it is to be noted that both of the diaphragms 96 and 97 are connected to the same terminal of the bias source 121, the outer plates 92 and 94 are each connected to the lead 117, and the intermediate plate 93 is connected to the lead 118.

By applying a signal at the terminals 113 and 114 the resulting forces upon the diaphragm 96, 97 are such that the diaphragms are distorted in opposite directions, either toward or away from each other. The principle is the same as that previously stated, that is, the bias and signal are additive between the diaphragm and plate on one side and subtractive on the other side. Thus, for one polarity of input signal the diaphragms 96, 97 are forced away from each other to increase the volume of the chamber therebetween, which includes the central apertured plate 93, and decrease the volume of the two outermost chambers of the stack. Air, or other fluid, is then pulled in at opening 126 and forced out at openings 127 and 128. A reversal of the applied signal results in the diaphragms 96, 97 being distorted toward each other so that the central volume is decreased and outer volumes increased. In the latter instance then air, or other fluid, is pulled in at openings 127 and 128 and forced out at opening 126. Because the diaphragms 96 and 97 are forced in opposite directions in the present embodiment of the invention the amount of air, or other fluid, displaced for a single input signal is greatly increased over any other arrangement.

It will be apparent then that the movement of the fluid in and out of opposite sides of the transducer 91 may be readily controlled by applying pulses of selected polarity to the two terminals 113 and 114. The transducer 91 may, for example, with suitable conduit connections be used for applying selective pneumatic pressures to control the movement of magnetic tape in passage between supply and takeup reels across magnetic transducers. In addition to the foregoing the transducer ,91 of FIGURES 5 and 6 may be used to sense pressure, as Well as pressure differential. For sensing pressure variations the electrical connections may be as shown in FIGURE 6 with the output being taken from the terminals 113 and 114 and with no applied alternating signal. For sensing pressure differenital between two sources of pressure, one source is coupled to the openings 127 and 128 and the second source is coupled to the oppositely disposed opening 126. The electrical connections may be the same as for the preceding application and, in both instances, the terminals 113 and 114 are connected to a measuring device or a utilization circuit, such as a servo loop to maintain the pressure constant.

Referring again to the embodiment of FIGURES 24, the frequency response of such a system where a large plurality of interleaved plates and diaphragms are used for high power applications, is improved in the manner illustrated in FIGURE 7. The connections to the diaphragms are the same as shown in FIGURE 3 and are not further illustrated here. The source of signals applied to the plates 121-127 is the same as in FIGURE 3 as connected to the terminals 57 and 58 and bears the same series of reference numerals.

In considering a large number of plates and diaphragms, or larger, as illustrated in FIGURE 7 it is to be realized that the mass of the diaphragms is analogous to a capacitor in an electrical analogue sense and, likewise, the compliance of air between the diaphragms is analogous to an inductance. To an audio wave commencing at one end and propagating through the laminar structure with the simple connections of FIGURE 3, the mass of the diaphragms and compliance of the air serves to delay the wave with respect to the exciting electrical signal so that at the output end of the stack improper phase relationships exist between the wave and the signal. The result is improper reproduction of the audio signals, particularly at the higher frequencies.

To overcome the foregoing where large stacks are required for high power audio output applications, the inherent electrical capacitance between the diaphragms is complemented by the addition of inductors 131 in the connecting leads between the terminals 57, 58 and the successive plates 121127. The result is an electrical delay line for the input signals to match the delay of the resultant air wave proceeding from the rear to the front, or output, of the laminar stack. Thus, the wave and signal arrive at the plates at the same time and prevent cancellations in the output. Such efiects are principally noticeable at the higher frequencies.

The present invention has been principally shown and described with respect to a laminar structure for an electrostatic transducer having two flexible diaphragms; however, it is to be realized that, while such arrangement increases the output power by a factor of two over a single diaphragm system, as the number of diaphragms is increased so is the output. Thus, a similar system having three diaphragms increases the output by a factor of three, a four diaphragm system increases the output by a factor of four, and so forth with the output increasing in direct proportion to the number of diaphragms.

In the foregoing there has been described in detail an improved electrostatic transducer of simple construction that has several different uses, which have been principally set forth as electrical-toacoustical conversion, acoustical-to-electrical conversion, electrical-to-pressure conversion, and pressure-to-electrical conversion. While the present invention has been set forth in detail with respect to structure for accomplishing such uses, numerous modifications and changes may be made Within the spirit and scope of the invention and it is not intended that the invention be limited to the exact details shown and described, except insofar as they may be defined in the following claims.

What is claimed is:

1. An electrostatic transducer assembly comprising a plurality of parallel mounted and apertured rigid plates; a similar plurality, less one, of flexible diaphragms respectively interleaved between the plates, the diaphragms being substantially coextensive with and parallel to the plates; a transformer having a center-tapped winding with two leads alternately connected to successive ones of the plates; a center-tapped source of bias having two electrodes of opposite polarity with successive diaphragms being alternately connected to the two electrodes and the center tap of the source coupled to the center tap of the transformer winding; and a transmission line having a time constant matching the inherent mechanical time constant of the plate and diaphragm combinations included between the two transformer leads and the successive plates.

2. The combination of claim 1 wherein the transmission line comprises series-connected inductances having values in combination with interplate capacitances to provide an electrical time constant matching the inherent mechanical time constant of the plate and diaphragm combination to movement of air' 3. An electrostatic transducer assembly comprising a plurality of parallel mounted and apertured rigid plates; a similar plurality, less one, of flexible diaphragms respectively interleaved between the plates, the diaphragms being substantially coextensive with and parallel to the plates; open-sided, electrically-insulating frames for supporting each of the apertured plates with the open side of successive frames oppositely disposed; cover plates disposed across the frames of each outermost plate of the plurality of plates; a transformer having a centertapped winding with two leads alternately connected to successive ones of the plates; and a source of bias having two electrodes of opposite polarity with one electrode connected to each of the diaphragms and the second electrode coupled to the center tap of the transformer winding.

References Cited in the file of this patent UNITED STATES PATENTS 1,082,248 Unterholzner Dec. 23, 1913 1,978,200 Heising Oct. 23, 1934 1,983,377 Kellogg Dec. 4, 1934 3,008,014 Williamson et a1 Nov. 7, 1961 FOREIGN PATENTS 1,083,302 France Jan. 7, 1955 838,024 Great Britain June 22, 1960 

1. AN ELECTROSTATIC TRANSDUCER ASSEMBLY COMPRISING A PLURALITY OF PARALLEL MOUNTED AND APERTURED RIGID PLATES; A SIMILAR PLURALITY, LESS ONE, OF FLEXIBLE DIAPHRAGMS RESPECTIVELY INTERLEAVED BETWEEN THE PLATES, THE DIAPHRAGMS BEING SUBSTANTIALLY COEXTENSIVE WITH AND PARALLEL TO THE PLATES; A TRANSFORMER HAVING A CENTER-TAPPED WINDING WITH TWO LEADS ALTERNATELY CONNECTED TO SUCCESSIVE ONES OF THE PLATES; A CENTER-TAPPED SOURCE OF BIAS HAVING TWO ELECTRODES OF OPPOSITE POLARITY WITH SUCCESSIVE DIAPHRAGMS BEING ALTERNATELY CONNECTED TO THE TWO ELECTRODES AND THE CENTER TAP OF THE SOURCE COUPLED TO THE CENTER TAP OF THE TRANSFORMER WINDING; AND A TRANSMISSION LINE HAVING A TIME CONSTANT MATCHING THE INHERENT MECHANICAL TIME CONSTANT OF THE PLATE AND DIAPHRAGM COMBINATIONS INCLUDED BETWEEN THE TWO TRANSFORMER LEADS AND THE SUCCESSIVE PLATES. 